In March 2003, he was loaned to Chesterfield for six weeks, in which time he played seven matches and scored once against Blackpool.[1] Ironically Blackpool were his next loan destination, joining them for three months, where he made a significant impact, playing in 16 league games and scoring three goals.

Despite being a tenacious central midfield, whose strengths focussed on energy and aggression rather than technical skill, Blackpool manager, Steve McMahon used the on-loan Douglas as an emergency left midfielder. This unexpected role was to prove a stroke of luck for Douglas. Not soon afterwards, Graeme Souness, manager of Blackburn Rovers, found his side struggling near the bottom of the Premier League and missing a left midfielder due to injuries and inadequate transfer dealings.

Jonathan Douglas was recalled by Rovers and promoted straight into the much re-jigged Rovers side, which adopted very defensive tactics to grind out results. The unheralded collaboration of Jonathan Douglas, Martin Andresen, Jonathan Stead, Lorenzo Amoruso and others, eventually proved successful in eking out results to guarantee Rovers safety in the Premier League. Douglas first Blackburn goal, a spectacular volley, came during this time, in a 4–3 victory over Fulham.[2]

Douglas had shown his potential as a utility squad player for Blackburn and Rovers awarded him a three-year contract.

However, manager Souness departed to Newcastle United and under new boss, Mark Hughes, Douglas soon fell down the pecking order and moved out on loan to Gillingham of the Championship, along with Rovers' youngster, Jay McEveley. Douglas was a regular in the side that so narrowly got relegated. However, he had once again impressed with his tenacious work-rate.

On 19 August 2005, he joined Leeds United on a season-long loan. He impressed in the early half of the season, making over 40 appearances for the club and scoring five goals in the process as Leeds reached the Championship playoff final but ended up losing 3–0 in the final against Watford. These performances impressed former manager Kevin Blackwell enough to make the move permanent, and on 31 August 2006, Douglas moved to Leeds permanently for an undisclosed fee, signing a three-year deal with the club.[3][4] Douglas' performances impressed new manager Dennis Wise and kept him in the Leeds starting lineup. Douglas' only goal for the club that season came in the 2–1 victory against Coventry City.

After Leeds' captain, Kevin Nicholls requested a transfer to Luton Town, Douglas was made captain by Wise. He had played numerous matches as Leeds captain previously due to injuries to Nicholls, Shaun Derry and Gary Kelly. Douglas was forced to sit out the rest of the season when he received his 15th yellow card in the defeat at Colchester United. Leeds' relegation that season saw many Leeds fans believing that Douglas would leave the club; it was reported that Douglas had a training ground bust up with Wise.[5] However, despite being stripped of the club captaincy, Douglas has been playing regularly in the Leeds first team in the 2007–08 season. Despite rumours of a transfer to Burnley, Douglas stayed at Elland Road. On 28 October 2007 Leeds defeated Millwall 4–2 with Douglas grabbing his second brace for the Yorkshire club in his two years at the club. A serious leg injury late on at Walsall ruled Douglas out for much of the remainder of 2007–08, an injury which coincided with Leeds' dip in form after their astonishing start to life in League One, as Leeds struggled to replace the running power and positional sense of the injured Irishman.

He returned to the squad on 22 March, playing the last ten minutes of the match against Walsall, a match Leeds won 2–0. Leeds reached the playoff final against Doncaster Rovers but lost the game 1–0. After Leeds' failure to win promotion to the Championship, Douglas was again linked with a move to Burnley. He scored the first goal in Leeds' 4–0 romping of Crystal Palace in the second round of the League Cup and again in the 5–2 league defeat of Crewe Alexandra. Douglas' played in a more unfamiliar position of right back as Leeds reached the playoffs again, only to be knocked out the competition when they lost 2–1 on aggregate to Millwall.[6]

On 21 May 2009 it was reported that Leeds would be offering him a new deal.[7] However the following day it was reported by official club text that Douglas, along with goalkeeper David Lucas, had been released from the club. This seemed to be down to a difference of opinion between club and player of the valuation of his services.[8]

On 10 July 2009, he signed for Swindon Town after being released from Leeds United.[9] On 10 October 2009, Douglas was given a straight red card in a match against Millwall.[10] Swindon felt the decision was unfair and prepared an appeal against the automatic three match ban for the dismissal. On 14 October the appeal was rejected and an extra one match ban added taking Douglas' suspension to four matches.[11] This incited chairman Andrew Fitton to launch a scathing attack on The FA and its justice system.[12] Douglas missed a home match with Hartlepool United, two away trips to Norwich City and Tranmere Rovers and the visit of Conference South side Woking in the FA Cup First Round. Douglas was part of the Swindon side which reached the League One playoffs. They won their playoff semi final after beating Charlton Athletic on penalties but were defeated by one goal to nil in the final at Wembley by Millwall. Douglas was later awarded both the Club's Player of the year award and the Club's Player's Player of the year award for the season 2009/2010. Douglas scored with a volley into the top corner from 25 yards out against Sheffield Wednesday in a 3–1 defeat, his only goal for the club.[13] However, Swindon were relegated to League Two after finishing bottom of the League One table in the 2010–11 season.

Douglas rejected an offer of a new contract at Swindon to sign for Brentford on 29 June 2011. At the start of the 2011/12, along with Richard Lee, he was named as a deputy to club captain Kevin O'Connor.

Douglas was named Brentford's Supporters' 'Player of the Year' for 2011/12,[14] having only missed 26 minutes of Brentford's League One season, scoring two goals and providing the assist for four others.

In the 2012/13 season Douglas played 52 games in total, as Brentford missed out on promotion after a dramatic final game against Doncaster Rovers. With O'Connor and Lee being restricted to the bench Douglas started the season captaining the side on many occasions. He scored his first goal of the season in a 1–1 draw away to Walsall in September and he also scored away to Tranmere Rovers later that month. He also scored against Doncaster, and Carlisle United

Douglas has earned international caps for the Republic of Ireland, the first against Poland on 28 April 2004. Having already featured in a number of friendlies, Douglas made his competitive debut for Ireland in the European Championship Qualifier against the Czech Republic on 11 October 2006, starting in central midfield alongside Lee Carsley. He also came on as a substitute in Ireland's next qualifier against San Marino. He was a regular choice in Steve Staunton's Republic of Ireland squad.